1. Field of the Invention
The invention relates to a method for modifying in a spatially periodic manner at least in some regions a surface of a substrate, said surface being disposed on a sample plane, for which end different regions of the substrate surface are acted upon successively with a spatially periodic illumination pattern of an energy density above a processing threshold of the substrate surface, wherein the illumination pattern is generated by diffraction of an input beam and superimposition of resulting, diffracted sub-beams by means of a grid interferometer, and wherein, in order to select the substrate surface region to be illuminated in each case, the input beam is deviated by means of a beam-deviating unit arranged upstream of the grid interferometer. The invention relates further to a device for modifying in a spatially periodic manner at least in some regions a surface of a substrate, said surface being disposed on a sample plane, comprising a beam-deviating unit to select a current processing region by controlled deviation of an input beam, and a grid interferometer arranged downstream from the beam-deviating unit, which interferometer bends the input beam and superimposes in the sample plane the resulting, diffracted sub-beams to create a spatially periodic illumination pattern in the current processing region.
2. Description of the Related Art
Methods and devices in the field of the invention are known from DE 10 2006 032 053 A1. Such devices and methods are used in areas such as security technology to equip documents, coins, bank notes, etc. with an anti-counterfeit security feature. When the correct pattern is selected, the structures implanted in the surfaces of films, coins, plastic carriers and other substrates produce defined optical effects, e.g. a reflection that is dependent on wavelength and/or direction.
For the generic method and/or generic device, an input beam, especially a laser beam, is directed through a grid interferometer to create the spatially periodic illumination pattern. For the purposes of this application, a grid interferometer is understood to be a device with at least two optical grids parallel to one another and which are essentially horizontally arranged with respect to an optical axis where an input grid is arranged so as to split an input beam into a plurality of sub-beams by diffraction, and an output grid is arranged so as to reconstitute in a sample plane a selection of the sub-beams created at the input grid or at other intermediate grids. The distance d between two adjacent grids is typically equal to the distance between the grid that is last following the direction of the beam and a sample plane P arranged further downstream with respect to the beam. The periods of the grids are in a specific proportion to one another that determines the orders of the usable sub-beams that interfere with one another and are superimposed in P. The following applies generally to the embodiment of the grid interferometer that is particularly relevant in practice, namely a two-grid interferometer with two grids G1 and G2 with corresponding grid periods p1 and p2: If the sub-beams of order n1 created by way of diffraction at G1 are again diffracted at G2 and the resulting sub-beams of order n2 are made to interfere and superimpose in P, the proportion of the grid periods should be p2/p1=n2/(2*n1). An expert would be familiar with the generalisation extending to grid interferometers with more than two grids. FIG. 1 illustrates the beam path of such a two-grid interferometer with n1=1 and n2=2, i.e. p1=p2. Suitable means of beam selection, e.g. apertures, are provided to select the correct sub-beams; for purposes of clarity they are not represented in FIG. 1.
A major advantage of the grid interferometer is its quality of forming the interference pattern independently of spatial and temporal coherence, wavelength, and incident angle of the input beam. In practice, it will be difficult to completely, i.e. perfectly, satisfy the above conditions regarding distances and grid periods; however, even under realistic conditions in practice, the aforementioned independence of the input beam's parameters remains true, at least to the relevant extent.
If the energy density of the illumination pattern is high enough to overcome a process threshold of the substrate surface, a corresponding permanent structure can be thus implemented within the substrate surface. The concept of process threshold is conceived broadly and encompasses, for example, a destruction threshold (e.g. for laser ablation) as well as energy thresholds for thermal or photochemical reactions.
Such process thresholds are generally relatively high, which means that strong focussing on the substrate surface is usually necessary. The cited category-defining document is therefore especially concerned with forming the input beam such that the energy density on the substrate surface is high enough to overcome the process threshold, yet also that the energy density in the region of the grid interferometer remains sufficiently low to as not to provoke destruction of the grid. The strong focussing ultimately means that the given region of the substrate surface that is illuminated, i.e. the current processing region, is comparatively small.
To create large-scale patterns, the cited document therefore suggests that a beam-deviating unit, such as a mirror scanner that deviates the input beam such that the illumination pattern can be shifted in a larger region of the substrate surface such that a large-scale structure can be successively created the substrate surface.
However, this approach is limited as the size of the region that can be achieved is highly restricted. The position of the focal plane of the input beam, especially its distance from the sample plane, varies with the deviation angle. The focus is on a spherical shell segment around the point of deviation. The image sharpness of the interference pattern, as well as the energy density with which the sample surface is hit, vary accordingly, and thus the processing efficiency varies depending on the diffraction angle.
A method and a device as described above are known from US 2006/0109532 A1, however without a beam-deviating unit. To process larger substrates, the substrates are positioned on a movable platform that is shifted after a sub-field has been processed such that a second field for processing is shifted into position at the place where the interference pattern is created.
U.S. Pat. No. 6,882,477 B1 also discloses a similar device without a beam-deviating unit and with a movable sample platform.
A method and device for laser tempering of polycrystalline semi-conductor layers are known from US 2004/0074881 A1. It relates to using a focussed laser beam to scan a processing surface. To do so, a collimated beam is deviated by a galvanometric mirror scanner wherein an F-Theta lens arranged downstream from the mirror scanner serves to focus the beam on the substrate surface independently of the deviation angle.
It is the task of the present invention to improve a generic method and a generic device such that it is possible to create large-scale patterns.